1. Field of the Invention
This invention lies in the field of cardiac assist procedures and, more specifically, to a system and method for enhancing the use of skeletal muscle in providing collateral blood flow to risk regions of the heart such as chronic ischemic myocardium.
2. Description of the Prior Art
There currently exists a class of cardiac patients who manifest angina, but for whom conventional medical therapy is not indicated, and who are not candidates for more severe procedures such as implantation of cardiac assist devices. Direct revascularization of ischemic mycardium with a coronary bypass to a large epicardial vessel is not always possible because of extensive distal coronary artery disease. Despite improvements in medical management, it has been found that about 5% of patients are considered inoperable, i.e., cannot undergo revascularization with angioplasty or surgery, because of distal disease. In addition, other patients who presently do undergo such palliative revascularization might be better served by a more effective indirect revascularization procedure.
Cardiomyoplasty, which involves wrapping the latissimus dorsi muscle around the left ventricle and stimulating the skeletal muscle to contract, is presently undergoing clinical trials in the United States under the guidance of the FDA. The mechanism of the augmentation of the cardiac function after cardiomyoplasty is thought to be primarily mechanical, resulting from a pincer-like compression of the left ventricle by the skeletal muscle. The skeletal muscle is stimulated by a cardiomyostimulator which delivers a series or burst of pulses to the skeletal muscle to evoke each contraction. It has also been known to use skeletal muscle to actually replace part of the heart or the aorta. Further, the skeletal muscle has been used to form a hydraulic pouch for cardiac assist, rather than to directly connect the muscle to the heart or the aorta. See Mannion, J. D. et al., "Hydraulic Pouches of Canine Latissimus Dorsi-Potential For Left Ventricular Assistance," 91(4) J. Thoracic & Cardiovasc. Surg. 534 (April 1986); U.S. Pat. No. 4,979,936, Stevenson et al., which is incorporated herein by reference. See also Mannion, J. D. et al., "Potential Uses of Skeletal Muscle For Myocradial Assistance," Surgical Clinics of North America, Vol. 65, No. 3, June 1985, which discusses several clinical applications for skeletal muscle in cardiac surgery.
However, in these procedures, the purpose is to provide mechanical assist, rather than supplying the myocardial tissue with sufficient blood to treat the angina. By contrast, the approach of this invention is to treat the angina by enhancing indirect revascularization of chronic ischemic mycardium.
Indirect revascularization of ischemic myocardium was an extensively studied topic before the advent of coronary artery bypass surgery. Eck and O'Shaunessey, among others, documented that myocardium could be supplied by extramyocardial collateral blood flow. However, the clinical usefulness of the concept of myocardial revascularization at the capillary level was never fully confirmed. While investigations did suggest that small amounts of blood from extramyocardial collaterals could perfuse myocardium, the total amount of extramyocardial blood flow was understood to be small. Because of the dramatic success of bypass surgery, research in indirect revascularization waned.
More recent observations in the area of coronary collateral circulation have stressed the importance of collaterals. In acute myocardial ischemia, myocardial collaterals have been implicated in improving regional myocardial function and survival rates after myocardial infarction, and in extending the time frame in which to perform successful reprofusion. The clinical evidence suggests that intracoronary collaterals, which are anatomically located in the endocardium, have an important functional role in many patients. Establishment of an additional extramyocardial source of collateral blood flow is anticipated to be helpful in relieving myocardial ischemia. However, as found by early investigators, the quantity of collateral blood flow from skeletal muscle is normally small.